A. Field of the Invention
The present invention relates to Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) compositions and associated methods.
B. Description of the Related Art
The swine industry continues to experience major economic losses due to PRRSV despite major efforts aimed at pathogen elimination and control strategies. PRRSV is an enveloped single-strand positive sense RNA virus classified under the order Nidovirales, family Arteriviridae and genus Arterivirus. The virus causes Porcine Reproductive and Respiratory Syndrome (PRRS), and has also been known as the Mystery Swine Disease and SIR. The symptoms of PRRS include reproductive failure in breeding females and respiratory disease in growing swine. The reproductive failure is associated with mid to late term abortions, increased numbers of mummified fetuses and weak born piglets and fewer healthy born piglets. PRRSV is also characterized by anorexia, fever and respiratory disease typically seen microscopically as interstitial pneumonia. Thus infection with PRRSV can have a significant negative effect on the productivity of a herd. It has been estimated that PRRSV costs the US swine industry around 560 million U.S. dollars a year. Compounding the situation is the fact that PRRSV is endemic in most swine producing countries and has a similar economic effect elsewhere.
Current control strategies to prevent the spread of the virus within and between herds include: the proper management of breeding animals, removal of infected individuals, strict biosecurity measures including air filtration systems, serum inoculation, and the use of vaccines. Both inactivated and modified (attenuated) live vaccines are commercially available. Inactivated vaccines produce a high level of safety but are generally considered as ineffective or as conveying a very limited degree of efficacy. Attenuated live vaccines have been widely used to reduce the transmission and clinical disease cause by wild type PRRS viruses. While modified live vaccine efficacy may vary against heterologous isolates, they remain the best vaccination option available to combat the disease.
Attenuated vaccines do carry some risk as several instances of suspected reversion to virulence have been reported. Therefore, new vaccines need to be capable of providing sufficient immunological protection to reduce transmission and clinical disease while also exhibiting a high degree of safety and resistance to genetic changes.
To increase genetic stability, viruses with cell passaged derived deletions in the nsp2 (nonstructural protein 2) region have been developed. The nsp2 protein is the largest PRRSV protein, is highly antigenic and possesses the highest genetic diversity of the PRRSV genome. The value of deletion mutants stems from the increased stability they confer. Shorter sequences result in less potential for reversion to virulence. Deleted portions would require insertions to revert back to its original state which is much less likely to occur than a simple substitution back to the original virulent form.